The present invention relates to a decision feeback equalizer (DFE) and, more particularly to a decision feedback equalizer capable of removing waveform distortions on a channel due to fading and interference from other radio communication systems automatically.
It is a common practice with a digital radio communication system to use a decision feedback equalizer for equalizing frequency selective fading. In a conventional equalizer of this type, a digital input signal come in through an input terminal is routed through a serial connection of a first to a third adder to a decision unit. The output of the decision unit is fed out as a digital output signal via an output terminal. At the same time, the digital output signal is applied to, among a first to a third shift register connected in series, the first shift register. The output of the first shift register is delivered to the second shift register and one input of a first multiplier. A first tap coefficient is fed to the other input of the first multiplier. The output of the first multiplier is fed to the third adder. The output of the second shift register is applied to a second multiplier which receives a second tap coefficient also. The output of the second multiplier is applied to the second adder. Further, the output of the third shift register is fed to a third multiplier which receives a third tap coefficient also. The output of the third multiplier is delivered to the first adder.
The problem with the conventional decision feedback equalizer is that all the adders have to be operated at the same time in order to insure equalization. Hence, when the transmission rate of digital data or the number of taps is increased, the operations of the adders cannot follow such a transmission rate or such a number of taps. Then, the equalizer is prevented from performing the expected function.